The degree of accuracy to be allowed to estimates of age founded on the number of 'annual' rings is, however, of secondary importance in comparison with the enormously greater hold on life possessed by trees as contrasted with the higher animals. Early in the nineteenth century the Swiss botanist A. P. de Candolle expressed the opinion that trees do not die from senile decay, but only as the result of injury or disease. Trees are constructed on a plan fundamentally different from that underlying the structure of the highly complex human organism, and are thus endowed with a sort of potential immortality. It has been suggested that some of the large corals in the Red Sea which are still tenanted by living polyps may have been growing in the days of the Pharaohs. The coral polyp represents the growing portion of a lifeless mass of rock which is constantly extended by the activity of the organism at the summit of each branch. Between a coral-reef and a tree there are many essential differences, but a rough analogy may be recognised. A tree, unlike the higher animals, does not reach a stage at which the whole of its substance attains a condition of permanence and fixity. It consists of a complex branching-system in which each shoot increases in length by virtue of the youthful vigour of its apex: to a large extent the tree as a whole consists of lifeless material incapable of further growth, as is the case of the older portions of a coral-reef; but the regular increase in girth of the trunk and its branches demonstrates that this comparison is only partially true, and that the power of growth in a tree is not confined to the extremities of the youngest shoots. The tip of every twig is composed of minute cells endowed with a potentiality of development like that which characterises the embryonic tissues of a seedling just emerged from the seed. In the course of its growth, each branch, by means of its living and dividing cells, contributes to the several parts of the complex mechanism of the tree. While the greater number of cells acquire a permanent form and lose the power of further development, there remains a cylinder of cells endowed with perpetual youth. This cylinder of living cells, known as the cambium, extends between the wood and bark from one end of the tree to the other: by its periodic activity it adds new layers of tissue each year and thus, by increasing the amount of conducting tubes for the transport of water and for the distribution of elaborated food, it enables the tree to respond to the increasing demands which are the necessary accompaniment of increasing size. It has already been pointed out that in the spring when the sap flows most vigorously the cambial cylinder produces larger tubes, and afterwards when the tree settles down to its normal life, these are succeeded by narrower and stronger tubes. These later formed elements serve also an important mechanical purpose; by the strength of their walls they